The recycling of post consumer plastics is rapidly growing in importance due to several factors: a) The increasingly growing cost of plastics as an effect of the oil price in the international markets, allow more advanced technologies to be adopted. b) Oil reserves apparently are on a clear decrease and world economies are struggling for conserving oil for strategic reasons. c) Most plastics are not biologically degradable, representing therefore an unsolvable problem on waste disposal. d) Tighter legislation is obliging producers and users to recycle most plastics.
On the other hand, automotive oil containers made out of ‘high density polyethylene’ (HDPE) present a unique challenge to recyclers. As oil attaches with molecular forces to its container, conventional washing or dilution have proved to be insufficient to provide an industrially fully reusable material, beyond sub-applications like fence-posts. Used oil containers can be neither washed by conventional processes, nor burned because of contamination generated by oil additives. Used oil containers represent a sizable and undesirable fraction of garbage landfills, as they cannot be easily compacted.
The California Integrated Waste Management Board “Project George” claims to have solved the specific problem of HDPE oil containers recycling (1998), although the process has not been disclosed and no patents have been found for it. The “Project George” process claims that it is a machine that processes the shredded plastic in a closed-loop system without the use of water, detergents, or chemicals that can cause the hazardous discharge of contaminants or additional waste by-products. With this claim, Project George's process would apparently differ from the presently claimed invention, which is based on a completely different process with the natural exception of the conventional shredding process, which otherwise is highly improved in the presently claimed invention for this specific purpose. U.S. Pat. No. 5,232,607 discloses a very high acceleration centrifugal process to directly remove oil strongly attached to oil container chips, whereas the present invention uses low speed centrifugation to remove plain water or small water-oil-detergent emulsion remnants after the main oil removal has already occurred by other means. U.S. Pat. No. 5,227,057 further describes the characteristics of a centrifuge designed to separate oil from plastic flakes without using a wash process or solvents. U.S. Pat. No. 5,711,820 claims a process using supercritical liquid CO2 at very high pressures to remove oil from HDPE contaminated containers. U.S. Pat. No. 5,316,591 further includes the use of high frequency cavitation of supercritical CO2 for the same purpose.
Just in California, some 16 million kilograms of high quality HDPE, coming from discarded oil containers are legally dumped in the landfills carrying along some 1.7 million gallons of harmful motor-oil. The first estimate for the whole United States points to two billion used oil containers and more than 10 million gallons of oil per year.
The primary criterion for plastic recycling is the purity of the end product. In the case of motor oil contamination, fractions in the order of 0.1 percent or more, of oil dissolved in the plastic, progressively impairs its full re-use as an industrial raw material for common products. The process described in the presently claimed invention allows the recuperation of plastic with oil impurities well under those limits.